When the sediment that is to be sampled is sandy, and saturated with water, the difficulty arises that the sample tends to fall out of the sample-holder as the sample-holder is being transported to the surface. Unless the sediment is quite cohesive--if the sediment is clay-ey, for example--the slightest knock or jar on the sample-holder can cause the sample of sediment to fall out of the holder.
One approach to the problem of gathering and containing a sample of sand, or the like, is as follows. The sample-holder may comprise a tube that is hammered down into the ground, by means of drive-rods from the surface. One way of removing the sample holder from the ground in such a case is to keep the tube attached to the bottom one of the drive rods, and to progressively unscrew the rods from each other as the sample is being brought up. But it is this act of unscrewing the lengths of drive rod from each other that leads to the troublesome jarring that shakes out the sample.
As an alternative, the tube may be provided with a cable: now, the rods are detached from the tube while the tube is still at the bottom of the hole. The rods can be withdrawn and unscrewed, and any jarring in that operation has no effect on the tube or the sample. Later, the tube may be pulled up--smoothly--by means of the cable.
A refinement to this manner of removing the tube, and the sample, from the hole is this. The tube is provided with a piston, which is a sliding fit in the tube. The cable is attached to the piston, not to the tube. The piston is positioned at the foot of the tube, with the tube in position at the bottom of the hole. The cable is now anchored (at the surface). As the tube is hammered down, the piston is prevented from travelling downwards, so that the piston in effect is caused to rise up the tube.
This action tends to make it possible for the sample to enter fully right up into the tube as the tube is hammered downwards. The piston is a very tight fit in the tube, so that the tension in the cable during hammering is considerable. The tightness of the piston is important, because as the tube is being withdrawn up the hole, the weight of the sample is being supported, at least partially, by the frictional grip of the piston to the tube.
It is important also that the piston be perfectly sealed in the tube, since any air leakage around the seal could destroy the suction effect, which again could allow the sample to fall out of the tube The piston, arranged in that manner, has been found to be very effective when retaining saturated sands, and other low-cohesion sediments.
It is recognized in the invention that there is a difficulty remaining with the anchored-piston technique just described, and that is that some sediments, though of low-cohesion, nevertheless require quite vigorous hammering of the tube in order to drive the tube down into the sediment.
This is a problem because the tube has to be relatively thin, and has to be made of soft material. The tube has to be thin and soft because the tube has to be cut open along its length in order to extract the sample from the tube. The invention is aimed at overcoming this difficulty.